The present invention relates generally to sine wave reconstruction techniques and more particularly to a method and circuitry for providing an output signal, representative of a sine wave source voltage, from corrupted voltages which appear on the lines connecting the source to a load.
There are a number of instances in which it is desirable to have an accurate representation of a sine wave source voltage. One of the more common of these is in the power conversion/motor control art where semiconductor (e.g., thyristor) converter bridges are used to power and control an electric motor. In these applications it is necessary to have an accurate representation of the source voltage in order to properly synchronize the rendering of the bridge semiconductors conductive with respect to the source voltage. This is normally done by detecting when the voltage crosses the zero axis.
If no disturbances occur on the lines, such detection poses no problems. Such, however, is seldom the case. This is particularly true where semiconductor power bridges are used and the transfer of current from one bridge thyristor to another causes periodic short circuits across the power lines which in turn causes rather severe disturbances on those lines. These disturbances become even more pronounced when plural converters are connected to the lines with the attendant increase in the number of disturbances. A number of systems are known and have been employed to reconstruct the sine wave representation of the source voltage. In U.S. Pat. No. 3,976,868 "Voltage Synthesization" by L. J. Lane, issued Aug. 24, 1976, signals proportional to the instantaneous values of the voltage at the terminals of the voltage supply and to the rate of change, with respect to the time, of the current supplied to a load are developed and utilized to synthesize the voltage. An elaboration of this scheme involves providing the output of the 3,976,868 patent to an overdriven amplifier, the output of which is then furnished to a self-tuning filter such as that described in U.S. Pat. No. 3,978,420 "Self-Tuning Filter" by L. J. Lane, issued Aug. 31, 1976. As a further example, reference is made to U.S. Pat. No. 4,399,395 "Line-to-Line Voltage Reconstruction for Synchronizing Thyristor Power Converter" P. M. Espelage, issued Aug. 16, 1983. In this latter patent the system described basically provides that the integral of the line-to-line voltage is summed with the product of a load current and the commutation inductance to provide a reconstructed source voltage representation.
These known methods are very satisfactory and properly employed furnish excellent results. They are, however, fairly complex and hence expensive and in those applications requiring a high degree of precision can be fully justified. There are, however, a number of situations in which this high degree of accuracy is not necessary and in which it is difficult to justify the expense of these systems.